Electric assist steering systems are known in the art. In electric assist steering systems, an electric assist motor, when energized, provides torque assist to aid the driver in turning steerable wheels of the vehicle. The electric assist motor is typically controlled in response to steering torque applied to the vehicle steering wheel and measured vehicle speed.
A controller monitors applied steering torque and vehicle speed and controls a drive circuit to control electric current applied to the electric assist motor. A drive circuit in an electric assist steering system may include field effect transistors (“FETs”) or other forms of solid state switching devices operatively coupled between the vehicle battery and the electric assist motor. Motor current may be controlled by pulse-width-modulation (“PWM”)of the FETs or switches.
An electric assist steering system may include a permanent magnet alternating current (PMAC) motor. One particular type of PMAC motor is a three-phase PMAC motor. In a three-phase PMAC motor, the FETs of the drive circuit are connected to provide three-phase voltage to the phases (A, B, C) of the motor. The phases (A, B, C) of the PMAC motor are connected at a common connection point referred to as the “star” of the motor. In operation of the three-phase PMAC motor, the A, B, and C phases of the motor are maintained 120 degrees (electrical) apart. For example, if phase A is at θ degrees, then phase B would be at (θ+120) degrees, and phase C would be at (θ−120) degrees.
In operation, the amount of torque produced by the PMAC motor is functionally related to the amplitude of the electric current in the phases of the motor. The frequency of the electric current in the phases of the PMAC motor is selected to create a magnetic field in the phase windings that rotates about the armature at a predetermined speed. The rotating magnetic field induces the rotor to rotate. The rotational speed of the rotor is thus determined by the frequency of the electric current in the phases of the PMAC motor.
In operation, the rotating magnetic field may be commanded to lead the rotor by some angle. The angle between the rotating magnetic field and the rotor is referred to as an advance angle. The advance angle of the electric current in the phases of the PMAC motor can be controlled by adjusting the phase angle of the current supplied to the windings. Typically, when controlling a PMAC motor, the advance angle is increased as the rotor speed increases.
During operation, a motor may experience what are commonly referred to as “motor faults.” Motor faults may occur as a result of a variety of physical or structural failures of the motor. For example, in a PMAC motor, motor faults may occur when there is a short in a motor phase, when there is a phase-to-phase short in the motor phases, or when there is an open in a motor phase. Motor faults may cause the electric current realized in the motor phases to be different than the current commanded to the phases. As a result, the motor may not achieve the desired rotational speed or provide the desired torque.
Also, during operation, faults may occur in the FET driver circuit or in a controller, such as an electronic control unit (“ECU”), that is used to control operation of the FET driver circuit in order to control the motor. Such faults in the FET driver circuit or in the ECU may also cause the electric current realized in the motor phases to be different than the current commanded to the phases.